Fruit setting and development normally depend on successful fertilization. In tomato and many other species, a major limiting factor for fruit setting is the extreme sensitivity of the pollen production to moderately high or low temperatures and inadequate humidity (Picken 1984). Parthenocarpy, which is the ability to set seedless fruits, enables to circumvent these environmental constraints on fruit production. Consequently, in many fruit bearing plants parthenocarpy is of considerable economic significance.
Parthenocarpy may either be artificially induced in normal plants or may occur naturally as the result of a genetic trait. Genetic parthenocarpy may be obligatory or facultative. In the former seeds are never formed. In facultative parthenocarpy seedless fruit is produced when environmental conditions are unfavorable for pollination and fertilization. However, when fertilization does occur seeded fruits are formed, which enables reproductive propagation of the facultative partienocarpic plants.
Parthenocarpy is discussed in various scientific articles, such as:
Abad M, Monteiro A A (1989) The use of auxins for the production of greenhouse tomatoes in mild-winter conditions: A review. Sci Hort 38: 167-192.
Gustafson F G (1939a) The cause of natural parthenocalpy. Amer J Bot 26: 135-138.
Gustafson F G (1939b) Auxin distribution in fruits and its significance in fruit development. Amer J Bot 26: 189-194.
Ho L C, Hewitt J D (1986) Fruit development. In: The Tomato Crop. A Scientific Basis for Improvement. (Athemon JR and Rudich J, Eds), Chapman and Hall, London, New York. Pp. 201-239.
Nitsch J P (1970) Hormonal factors in growth and development. In: The biochemistry of fruits and their products (AC Hulme ed). Vol 2, Academic Press, London. Pp.428-47.
Picken A J (1984) A review of pollination and fruit set in the tomato (Lycopersicon esculentum Mill .) J Hort Sci 59: 1-13.
Varga A, Bruinsma J (1986) Tomato. In: CRC Handbook of Fruit Set and Development (Monselise SP, ed.) CRC Press Inc. Boca Raton, Fla. Pp. 461-481. In the tomato, the most effective treatments for induction of parthenocarpy utilize auxins, synthetic auxins or auxin transport inhibitors (reviewed by Ho and Hewitt 1986, Varga and Bruinsma 1986, Abad and Monteiro 1989). However, this treatment is very laborious since the auxin has to be applied to each truss separately, to avoid the adverse effects characteristic to auxin application to the whole plant.
Auxin also appears to play a role in genetic (natural) parthenocarpic fruit set. For example, Gustafson (1939a,b) found that auxin concentrations in ovaries of parthenocarpic orange, lemon and grape varieties were significantly higher than in seeded varieties. Based on similar data, Nitsch (1970) suggested that natural parthenocarpy is related to the ability of seedless varieties to establish a threshold concentration of hormones required for fruit set at anthesis.
Several genes which control the effect of auxin in plants are known in the art.
One such gene, for example, is the rolB gene which is included in TL-DNA of the Agrobacterium rhizogenes agropine-type Ri-Plasmid. (Chilton M D, Tepfer D A, Petit A, David C, Casse-Delbart F, Tempe J (1982) Agrobacterium rhizogenes inserts T-DNA into the genome of host plant root cells. Nature 295:432-434. Spano L, Pomponi M, Constantino P, van Sloteren G M S and Tempe J (1982) Identification of T-DNA in the root-inducing plasmid of the agropine type Agrobacterium rhizogenes 1855. Plant Mol Biol 1: 291-300, White F F, Ghidossi G, Gordon M P, Nester E W (1982) Tumor induction by Agrobacterium rhizogenes involves the transfer of plasmid DNA to the plant genome. Proc. Natl Acad Sci USA 79:3193-3197, Willmitzer L, Sanchez-Serrano J, Buschfeld E, Schell J (1982) DNA from Agrobacterium rhizogenes is transfered to and expressed in axenic hairy root plant tissues. Mol Gen Genet 186:16-22., Peterson SG, Stummann B M, Olsen P, Henningsen K W (1989) Structure and function of root-inducing (Ri) plasmids and their relation to tumor-inducing (Ti) plasmids. Physiol Plant 77:427-435).
Transgenic plants expressing the rolB gene under its native or constitutuve promoter manifest several syndromes characteristics of auxin toxicity, such as leaf abnormalities, increased stigma and flower size, heterostyly and increased formation of adventitious roots on the stem (Schmulling T, Schell J, Spena A (1988) Single genes from Agrobacterium rhizogenes influence plant development EMBO J 7:2621-2629). van Altvorst et al. (van Alvorst A C, Bino R J, van Dijk A J, Lambers A M J, Lindout W H, van der Mark F, Dons J J M (1992) Effects of the introduction of Agrobacterium rhizogenes rol genes on tomato plant and flower development. Plant Sci 83:77-85.) described the phenotypic effects of the various rol genes in transgenic tomato, however, they did not report parthenocarpic fruit development when the rolB gene was inserted.